The present invention relates to a new and improved method for fabricating precision x-ray collimators including precision focusing x-ray collimators.
X-ray collimators are devices that select parallel, divergent or convergent rays from an uncollimated source. Collimators are used in nuclear medicine and x-ray imaging to improve spatial resolution and sensitivity of the imaging system. A typical imaging system consists of a point radiation source and an image recording device, the object to image being placed between the radiation source and the detector. As the radiation interacts with the tissue, the radiation becomes attenuated as well as scattered by the tissue. Without intervention, both the scattered radiation and primary radiation from the patient are recorded in a radiographic image. Subject contrast and the signal to noise ratio of details in the image are reduced. In some types of x-rays, the presence of scatter can cause up to a 50% reduction in contrast and up to a 55% reduction in signal to noise ratio. It is important therefore to be able to fabricate collimators that permit the primary radiation to pass through, while attenuating or eliminating the scattered radiation.
A key problem is the need for high resolution and improved image quality in nuclear medicine and x-ray imaging. In nuclear medicine imaging, often more than 99% of the incoming photon flux is absorbed by the collimator, in exchange for the best spatial resolution provided by the particular hole-shape and hole pattern of the collimator in use. As a result, the photon statistics, and hence the image quality, is very limited. Conventional techniques for manufacturing collimators have great limitations on the hole-shape, hole pattern, and septa thickness that can be produced, which in turn results in relatively poor resolution and image quality. Typical spatial resolutions encountered in nuclear medicine imaging currently range from a few millimeters to centimeters, pixel count uncertainty can be worse than 30%, and the overall quantitative inaccuracy can be worse than 25%. If a sub-millimeter spatial resolution can be achieved, and quantitative measurements can be certain within 5%, the clinical utility of nuclear medicine imaging methods can be greatly expanded with high diagnostic accuracy. Similar situations exist in x-ray imaging, beta-ray imaging, and other radiological imaging techniques that use collimator devices to achieve or improve spatial resolution and image quality.
A number of methods have been suggested for fabricating collimator devices. For example, U.S. Pat. Nos. 4,288,697; 4,951,305; 5,099,134; 5,231,655; and 5,303,459 describe various methods for fabricating collimators. Typically the anti-scatter grids are one-dimensional arrays of lead lamella, sandwiched between more x-ray transparent spacer materials, such as aluminum, carbon fiber or wood.
A need exists for a new and improved method for fabricating precision x-ray collimators.
A principal object of the present invention is to provide a new and improved method for fabricating precision focusing x-ray collimators.
It is another object of the present invention to provide such an improved method for fabricating precision focusing x-ray collimators that utilizes LIGA (German abbreviation of three major process steps, lithography, electroplating and molding) fabrication methods along with a synchrotron radiation from an electron storage ring, such as the Advanced Photon Source (APS) at Argonne National Laboratory.
It is another object of the present invention to provide such an improved method for fabricating precision focusing x-ray collimators that includes enhanced capabilities to move the substrate during exposure during LIGA.
In brief, a method is provided for fabricating precision x-ray collimators including precision focusing x-ray collimators. Fabricating precision focusing x-ray collimators includes the steps of using a substrate that is electrically conductive or coating a substrate with a layer of electrically conductive material, such as a metal. Then the substrate is coated with a layer of x-ray resist. An intense collimated radiation source is utilized for exposing the layer of x-ray resist with a pattern of x-ray. The pattern delineates a grid of apertures to collimate the x-rays. Exposed parts of the x-ray resist are removed. Regions of the removed x-ray resist are electroplated. Then remaining resist is optionally removed from the substrate.
In accordance with features of the invention, when exposing the layer of x-ray resist with a pattern of x-ray for non-focusing collimators, the substrate is maintained perpendicular to impinging x-rays from the synchrotron radiation source; and the substrate is scanned vertically. For precision focusing x-ray collimators, the substrate is scanned vertically in the z-direction while varying the angle of inclination of the substrate in a controlled way as a function of the position of the z-direction during the scan.